1. Field of the Invention
The present invention relates to resin recycling, and more particularly to a resin type identification method and a resin type identification apparatus featuring a method of selecting an infrared reflection spectrum to be used when recycled resin is identified according to composition using an optical method.
2. Description of the Related Art
When resin from used household appliances is recycled, parts in which the resin can be dismantled by hand are limited. Therefore, small components, components having a complicated configuration, and so on must be pulverized by machine and then sorted into metal, resin, and so on in order to be turned into recycled material.
In this case, the respective materials must be separated from a pulverized, intermixed condition, and therefore a sophisticated sorting technique is required. Metal is sorted by specific gravity or using electric or magnetic force. Resin, however, cannot be sorted using electric or magnetic force, and therefore classification according to specific gravity, electrostatic charge, and so on has been proposed.
However, it is difficult to identify similar types of resin using these methods. Hence, an identification method that focuses on differences in an absorptivity or a wavelength (wave number) dependence of a reflectance of the resin in relation to light in a near-infrared band or a mid-infrared band has been proposed.
Here, when black resin containing carbon black or the like is identified, a large amount of absorption occurs in the near-infrared band, and therefore a required signal strength cannot be obtained, making identification difficult. Hence, to identify black resin, it is preferable to use the mid-infrared band, which is less affected by the absorption of the carbon black.
In a method (an infrared spectroscopy method) of identifying individual pulverized resin pieces using the mid-infrared band, samples are conveyed in succession by a conveyor, and the samples are measured from above by a reflection method using FT-IR (Fourier Transform-Infrared Spectroscopy) (see Japanese Patent Application Publication No. S60-089732 and Japanese Patent Application Publication No. H8-300354, for example).
Note that typically, when identifying resin pieces using FT-IR, positioning between a measurement optical system and a measurement subject resin piece (to be referred to hereafter simply as “positioning”) must be performed with a high degree of precision to ensure that a measurement region of the measurement optical system is positioned on a surface of the measurement subject resin piece (to be referred to hereafter as “sample”).
The reason for this is that when the position of the sample is offset, a proportion of the measurement region occupied by the sample decreases, leading to a reduction in an infrared reflection intensity from the sample and intermixing of infrared reflection from a sample stage. Furthermore, as a result, an S/N ratio (a magnitude of noise relative to a peak height generated by the resin) of an infrared reflection spectrum generated by the sample decreases, and therefore the resin piece cannot be identified accurately.
More specifically, a size of the measurement region when FT-IR is performed using a specular reflection method or a diffuse reflection method is typically between approximately several mm and 1 cm. Therefore, when the size of the sample is equal to or smaller than 1 cm, an offset of several mm in the sample leads to an error during identification of the resin piece. Note that the size of the measurement region is dependent on a size of a light source, a size of a light reception unit of a detector, and the optical system.
However, following problems arise in the related art.
When, in the identification method using FT-IR according to the related art described in Japanese Patent Application Publication No. S60-089732 and Japanese Patent Application Publication No. H8-300354, a plurality of samples are separated and conveyed individually through the optical measurement system, it is impossible to identify resin pieces accurately unless the individual samples are positioned with a high degree of precision.
In the related art, it is difficult to convey the samples individually when the samples are smaller than the measurement region. It is also difficult to position the individual samples in a short amount of time. As a result, a large amount of time is required to identify the resin pieces accurately.
Further, in an identification method using near-infrared light or Raman light, a plurality of array sensors are arranged in a single row, and therefore the need for positioning can, in effect, be eliminated. In the identification method using FT-IR, however, an interference optical system is used, making it more difficult to arrange a plurality of detectors in a single row than with an identification method using near-infrared light or Raman light.
Hence, the following three methods are typically cited as methods of positioning samples using a single detector.
(1) A method of moving individual samples to a measurement region range by adjusting each sample in two directions, namely an X direction and a Y direction.
(2) A method of adjusting the individual samples only in the Y direction and then moving each sample in the X direction using a conveyor belt or the like so that the sample arrives at the measurement region range at a predetermined timing.
(3) A method of moving the individual samples in the X direction and the Y direction so that each sample arrives at the measurement region range at a predetermined timing.
Here, the method of (1) is the most suitable method for shortening a time interval between the end of measurement of one sample and the start of measurement of the next sample. However, a large-scale device is required to adjust samples having different shapes and sizes in two directions, i.e. the X direction and the Y direction. Further, in the method of (3), the time interval is longest, and therefore efficiency is poorest.
Hence, in an example of a specific operation executed when positioning is performed using the method of (2), a plurality of samples are carried on a metallic moving stage (or a conveyor belt or the like) so as not to overlap and moved in succession, and the samples are aligned by a position adjustment rail or the like disposed in a perpendicular direction to a movement direction of the samples, for example. A timing at which each sample arrives at the measurement range is detected by a position sensor or the like, and measurement is performed at this timing.
In another example of a specific operation, an identification unit performs measurement while the aligned samples move, regardless of whether or not a sample is present, whereby infrared reflection spectra are obtained continuously at fixed intervals. One or a plurality of infrared reflection spectra including an infrared reflection signal from a sample are then selected from the obtained infrared reflection spectra, whereupon the selected infrared reflection spectrum is used to identify the resin piece.
When a plurality of small resin pieces are identified automatically in succession, it is difficult to position the individual resin pieces, and therefore the latter method is preferable. With the latter method, however, in which one or a plurality of infrared reflection spectra is selected, it has not been possible up to the present to clarify a reference on the basis of which to select an optimum spectrum for identifying the resin pieces, and therefore, when an infrared reflection spectrum which is unsuitable for identification is selected, identification cannot be performed accurately, leading to a determination error.
Here, when a spectrum simply having a high infrared reflection intensity is selected, the selected spectrum may be a spectrum having a poor S/N ratio due to intermixing of a strong reflection spectrum from the moving stage, and in this case, accurate identification is difficult. When, on the other hand, a spectrum having the lowest infrared reflection intensity is selected to avoid selection of a spectrum intermixed with an infrared reflection spectrum from the moving stage, the intensity of the selected spectrum may be too low, making accurate identification difficult.